1. Field of the Invention
This invention relates to a process for preparing an optically active mercapto compound having the following formula [A]: ##STR2## in which R.sub.1 represents an alkyl group, R represents a hydrogen atom or an alkyl group, C* represents an asymmetric carbon, n represents the value of 1 or 2.
The optically active mercapto compound prepared by the process of this invention is useful as a raw material for producing a medicinal compound, and specifically for producing a compound having medicinal effects such as antihypertensive activity.
2. Prior Art and Background of the Invention
Conventionally known processes for preparing the compound represented by formula [A] are (1) a process utilizing an optical resolving reagent (Japanese Patent Laid-open Nos. 151,912/1979 and 38,386/1980), and (2) a process for preparing this compound by reacting an optically active halogenated compound with a hydrosulfide which is obtained from hydrogen sulfide and a base (Japanese Patent Laid-open No. 38,768/1982).
The process utilizing an optical resolving reagent, however, requires a large amount of expensive resolving reagent for preparing optically active mercapto compound. The process has further disadvantages in that the product tends to contain the resolving reagent as an impurity and the resolving step is complicated. Thus, this is not necessarily a satisfactory process for industrially producing optically active mercapto compounds. The process disclosed in Japanese Patent Laid-open No. 38,768/1982 requires an optically active halogenated compound as an essential raw material, and cannot be applied where a mercapto compound is used as a raw material.
There are also known in the art processes for preparing optically active carboxylic acids or asymmetric esters by the action of enzymes, culture broth of microorganisms, bacterium, or bacterium-treated substances (Japanese Patent Laid-open No. 94,091/1985, Japanese Patent Laid-open No. 141,297/1985, and Japanese Patent Laid-open No. 256,391/1985). These processes, however, require a procedure for protecting thiol groups. In case where the intended product is a optically active mercapto carboxylic acid, the processes further requires, in addition to the procedure for protecting thiol groups, a step for eliminating the thiol groups. These drawbacks make it difficult to produce such optically active compounds inexpensively and efficiently.
The present inventors have made extensive studies on the process for preparing either d- or .tau.-isomer of carboxylic acid, or asymmetric carboxylic acid esters by asymmetrical hydrolysis of mixtures of carboxylic acids having d- and .tau.-isomers of mercapto groups, using fermentation engineering technique and with industrially acceptable efficiency. As a result, the inventors found that an optically active mercapto compound could be prepared with good efficiency without protecting free mercapto groups which is usually unstable in the reaction conditions.